The invention relates to a hydraulic control device according to the preamble of claim 1 and a to a pressure switch according to the preamble of claim 15.
Hydraulic control devices containing a pressure switch are known in practice in various embodiments. In one embodiment (FIG. 1—prior art) the pressure switch is located within a main flow path between the pressure source and the valve assembly. The pressure switch is a 3/2-way slider valve and connects in one control position the pressure source with the valve assembly and separates the valve assembly from the reservoir. In the other control position the connection to the pressure source is blocked and the valve assembly is connected with the reservoir. In order to achieve a smooth movability of the control member of the slider valve the control member needs a slide fit which causes unavoidable and undesirable leakage losses in flow direction to the reservoir. These leakage losses are detrimental e.g. in the case of a pump having a small displacement volume employed to build up high supply pressure in the system with a small flow rate only. In such control devices leakage losses occurring when the pressure source is driven cannot be tolerated. Among others the main reason for using the pressure switch in such hydraulic control devices is that after switching off the pressure source the pressure acting at the valve assembly has to be relieved to the reservoir, e.g. in order that the drive motor of the pump constituting the pressure source does not have to operate instantaneously against relatively high residual resistance when the drive motor is switched on. For example a one phase alternating current motor could hardly start against counter pressure. This needs to use an excessively strong and costly drive motor which is able to start properly despite residual counter pressure. In order to avoid starting problems of a drive motor just strong enough to deliver sufficient power to provide the needed torque for a small flow rate at elevated motor speed, it is furthermore known to equip the pressure source with an auxiliary volume in a pilot pressure channel. The pilot pressure channel directs the pilot pressure actuating the control member in the pressure switch in a first switching direction. The auxiliary volume is defined in a chamber within which a piston yields against spring force such that then when the drive motor starts while the pressure switch separates the valve assembly from the reservoir and connects the valve assembly with the pressure source, the pressure source is first filling the auxiliary volume with the result that the drive motor first only has to overcome small resistance. The auxiliary volume, however, means additional structural measures and allows to achieve the desired function for the motor start only if the maximum pressure in the system does not exceed e.g. about 300 bars. In case of higher maximum pressures of e.g. up to 700 bars, however, the auxiliary volume no longer functions satisfactorily.